The goal of this proposal is to determine if MUC18 enhances pro-inflammatory responses to viral and bacterial infections in allergic lungs. MUC18 is a transmembrane glycoprotein that is normally expressed on endothelial cells and smooth muscle cells. We have generated highly novel data linking MUC18 to the elaboration of pro- inflammatory cytokines in airway epithelial cells and alveolar macrophages that are exposed to IL-13 (a key Th2 cytokine in asthma). We identified a significant increase of MUC18 in human asthmatic airway epithelial cells and alveolar macrophages, and induction of MUC18 in these cell types by IL-13. By using MUC18 knockout, knockdown or over-expression approaches, we discovered that MUC18 enhances airway epithelial and lung macrophage pro-inflammatory cytokine (e.g., IL-8 and TNF-? production in response to human rhinovirus, various strains of bacteria and Toll-like receptor (TLR) agonists. Importantly, MUC18 knockdown in IL-13-exposed mouse lungs significantly reduced inflammation (neutrophils and chemokine KC) following bacterial infection. Finally, we have identified MUC18 specific intracellular phosphorylation sites that regulate its function. Our innovative research in MUC18 leads to the hypothesis that MUC18 up-regulation in allergic lungs amplifies inflammatory responses to various pathogens, leading to asthma exacerbations. Aim 1 will determine the mechanisms of MUC18 up-regulation by pathogens in allergic lungs. By using primary human airway epithelial cells and lung macrophages, mouse models of IL-13 exposure and viral/bacterial infection, and gene knock-down and overexpression strategies, we will test if pathogen infection activates mitogen-activated protein kinases through TLR signaling, which subsequently increases the activity of transcription factor specificity protein 1, and amplifies MUC18 up-regulation in Th2 cytokine-exposed lung cells. Aim 2 will define the pro-inflammatory function of MUC18 up-regulation during viral and bacterial infection. By using primary human cell cultures, mouse models of allergen exposure and viral/bacterial infection, and MUC18 conditional knockout mice, we will test the hypothesis that MUC18 up-regulation in allergic lungs promotes pro-inflammatory responses to pathogen infection. Aim 3 will dissect the mechanisms of MUC18 pro-inflammatory function. By using MUC18 mutants and transgenic mice over-expressing human wild-type MUC18 and a human MUC18 mutant, we will test the hypothesis that following pathogen-mediated activation of mitogen-activated protein kinases, MUC18 cytoplasmic tail is phosphorylated, resulting in NF-?B activation and amplified production of pro-inflammatory cytokines (e.g., IL-8 and eotaxin). Completion of our proposed studies will greatly improve our understanding about MUC18 pro-inflammatory function during various viral and bacterial infections in asthma. Our research findings will provide new and broad strategies (e.g., anti-MUC18 therapy) to prevent and treat acute exacerbations of asthma.